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        <p>nio</p>
<a id="more"></a>
<h1 id="Selector-实现原理"><a href="#Selector-实现原理" class="headerlink" title="Selector 实现原理"></a>Selector 实现原理</h1><p>linux下select的实现是EpollSelectorImpl</p>
<p>EPollArrayWrapper将Linux的epoll相关系统调用封装成了native方法供EpollSelectorImpl使用。 </p>
<figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="function"><span class="keyword">private</span> <span class="keyword">native</span> <span class="keyword">int</span> <span class="title">epollCreate</span><span class="params">()</span></span>;</span><br><span class="line"><span class="function"><span class="keyword">private</span> <span class="keyword">native</span> <span class="keyword">void</span> <span class="title">epollCtl</span><span class="params">(<span class="keyword">int</span> epfd, <span class="keyword">int</span> opcode, <span class="keyword">int</span> fd, <span class="keyword">int</span> events)</span></span>;</span><br><span class="line"><span class="function"><span class="keyword">private</span> <span class="keyword">native</span> <span class="keyword">int</span> <span class="title">epollWait</span><span class="params">(<span class="keyword">long</span> pollAddress, <span class="keyword">int</span> numfds, <span class="keyword">long</span> timeout,</span></span></span><br><span class="line"><span class="function"><span class="params">                                 <span class="keyword">int</span> epfd)</span> <span class="keyword">throws</span> IOException</span>;</span><br></pre></td></tr></table></figure>
<p><img src="http://incdn1.b0.upaiyun.com/2017/08/f927dbf90bf79752d25ca2dcc6e5da7e.png" alt="img"> </p>
<h2 id="epoll原理"><a href="#epoll原理" class="headerlink" title="epoll原理"></a>epoll原理</h2><p>epoll是Linux下的一种IO多路复用技术，可以非常高效的处理数以百万计的socket句柄。<br>先看看使用c封装的3个epoll系统调用:</p>
<ul>
<li>int epoll_create(int size)</li>
</ul>
<p>epoll_create建立一个epoll对象。参数size是内核保证能够正确处理的最大句柄数，多于这个最大数时内核可不保证效果。</p>
<ul>
<li>int epoll_ctl(int epfd, int op, int fd, struct epoll_event *event)</li>
</ul>
<p>epoll_ctl可以操作epoll_create创建的epoll，如将socket句柄加入到epoll中让其监控，或把epoll正在监控的某个socket句柄移出epoll。</p>
<ul>
<li>int epoll_wait(int epfd, struct epoll_event *events,int maxevents, int timeout)</li>
</ul>
<p>epoll_wait在调用时，在给定的timeout时间内，所监控的句柄中有事件发生时，就返回用户态的进程。</p>
<p>大概看看epoll内部是怎么实现的：</p>
<ol>
<li>epoll初始化时，会向内核注册一个文件系统，用于存储被监控的句柄文件，调用epoll_create时，会在这个文件系统中创建一个file节点。同时epoll会开辟自己的内核高速缓存区，以红黑树的结构保存句柄，以支持快速的查找、插入、删除。还会再建立一个list链表，用于存储准备就绪的事件。</li>
<li>当执行epoll_ctl时，除了把socket句柄放到epoll文件系统里file对象对应的红黑树上之外，还会给内核中断处理程序注册一个回调函数，告诉内核，如果这个句柄的中断到了，就把它放到准备就绪list链表里。所以，当一个socket上有数据到了，内核在把网卡上的数据copy到内核中后，就把socket插入到就绪链表里。</li>
<li>当epoll_wait调用时，仅仅观察就绪链表里有没有数据，如果有数据就返回，否则就sleep，超时时立刻返回。</li>
</ol>
<p>epoll的两种工作模式：</p>
<ul>
<li>LT：level-trigger，水平触发模式，只要某个socket处于readable/writable状态，无论什么时候进行epoll_wait都会返回该socket。</li>
<li>ET：edge-trigger，边缘触发模式，只有某个socket从unreadable变为readable或从unwritable变为writable时，epoll_wait才会返回该socket。</li>
</ul>
<p>socket读数据</p>
<p><img src="http://incdn1.b0.upaiyun.com/2017/08/bc7d7b6c3b3589e8a522196f6de4cfc8.png" alt="img"> </p>
<p>socket写数据 </p>
<p><img src="http://incdn1.b0.upaiyun.com/2017/08/0b18b383d760281fecb46b4993c6de95.png" alt="img"> </p>
<p>顺便说下在Linux系统中JDK NIO使用的是 LT ，而Netty epoll使用的是 ET。 </p>
<h1 id="NIO比BIO效率高原因"><a href="#NIO比BIO效率高原因" class="headerlink" title="NIO比BIO效率高原因"></a>NIO比BIO效率高原因</h1><p>网上大多给出了两者的区别，可是具体效率高在哪里呢。</p>
<p>首先我们看一下各自的特点<br><strong>BIO：</strong></p>
<ol>
<li>socketServer的accept方法是阻塞的。</li>
<li>当有连接请求时，socketServer通过accept方法获取一个socket</li>
<li>取得socket后，将这个socket分给一个线程去处理。此时socket需要等待有效的请求数据到来后，才可以真正开始处理请求。</li>
<li>socket交给线程后，这时socketServer才可以接收下一个连接请求。</li>
<li>获得连接的顺序是和客户端请求到达服务器的先后顺序相关。</li>
</ol>
<p><strong>NIO：</strong></p>
<ol>
<li>基于事件驱动，当有连接请求，会将此连接注册到多路复用器上（selector）。</li>
<li>在多路复用器上可以注册监听事件，比如监听accept、read</li>
<li>通过监听，当真正有请求数据时，才来处理数据。</li>
<li>不会阻塞，会不停的轮询是否有就绪的事件，所以处理顺序和连接请求先后顺序无关，与请求数据到来的先后顺序有关</li>
</ol>
<h2 id="主要对比"><a href="#主要对比" class="headerlink" title="主要对比"></a>主要对比</h2><p>这里写的有点问题，NIO处理的时候也需要多个线程的，针对2，那个7秒对java来说是不存在的</p>
<ul>
<li><p>BIO一个连接，一个线程，非http请求，有可能只连接不发请求数据，此时线程是无用浪费的。</p>
</li>
<li><p>BIO处理依赖于连接建立；NIO处理依赖于请求数据的到来。导致执行顺序不同。</p>
<ol>
<li>一个线程处理一个请求<br><strong>BIO</strong>：连接请求来，建立socket，等待请求数据到来（t1），处理时间（t2）<br><strong>NIO</strong>：连接请求来，注册到selector，设置读监听，等待请求数据（t1），处理时间（t2）<br>此时，两者用时皆为t1+t2，没有区别</li>
<li>一个线程处理两个请求<br>第一个请求，等待请求数据（10），处理时间（1）<br>第二个请求，等待请求数据（1），处理时间（2）<br><strong>BIO</strong>：用时 10+1+1+2=14，第1个执行完用时10+1，等待第一个执行完处理第2个，用时1+2<br><strong>NIO</strong>：用时 1+2+7+1=11， 第二个数据先到，时间 1+2，此时第一个需要等时为10秒，还没到，还需等待7秒，时间为7+1</li>
<li>两个线程处理两个请求<br>第一个请求，等待请求数据（10），处理时间（1）<br>第二个请求，等待请求数据（1），处理时间（2）<br><strong>BIO</strong>：用时 10+1+2=13，等待第1个请求10，交给工作线程一处理，此时同时接受第2个，等待1秒，处理时间2秒，此间线程一处理时间为一秒，在线程二结束之前就已经结束<br><strong>NIO</strong>：用时 1+2+7+1=11，第二个数据先到，时间 1+2，此时第一个还没到，还需等待7秒，时间为7+1<br><strong>如果两个请求顺序相反，则bio和nio一样，都是11秒</strong><br>由此可见由于阻塞等待机制的不同，导致效率不同，主要优化点为，不必排队等待，先到先处理，就有可能效率高一点。</li>
</ol>
</li>
<li><p>BIO如果想要处理并发请求，则必须使用多线程，一般后端会用线程池来支持<br>NIO可以使用单线程，可以减少线程切换上下文的消耗。<br>但是虽然单线程减少了线程切换的消耗，但是处理也变为线性的，也就是处理完一个请求，才能处理第二个。<br>这时，有这么两个场景：</p>
<ol>
<li>后端是密集型的计算，没有大量的IO操作，比如读些文件、数据库等</li>
<li>后端是有大量的IO操作。</li>
</ol>
<p>当为第一种场景时：<br>NIO单线程则比较有优势， 理由是虽然是单线程，但是由于线程的计算是并发计算，不是并行计算，说到底，计算压力还是在CPU上，一个线程计算，没有线程的多余消耗，显然比NIO多线程要高效。BIO则必为多线程，否则将阻塞到天荒地老，但多线程是并发，不是并行，主要还是依靠CPU的线性计算，另外还有处理大量的线程上下文。<br>如果为第二种场景，多线程将有一定优势，多个线程把等待IO的时间能平均开。此时两者区别主要取决于以上分析的处理顺序了，显然NIO要更胜一筹。</p>
</li>
</ul>
<h2 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h2><p>NIO在接收请求方式上，无疑是要高效于BIO，原因并非是不阻塞，我认为NIO一样是阻塞的，只是方式不同，先来的有效请求先处理，先阻塞时间短的。此时间可用于等待等待时间长的。<br>在处理请求上，NIO和BIO并没有什么不同，主要看线程池规划是否和理。NIO相对BIO在密集型计算的模型下，可以用更少的线程，甚至单线程。</p>
<p>原文地址</p>
<p>Selector 实现原理 <a href="http://www.importnew.com/26258.html" target="_blank" rel="noopener">http://www.importnew.com/26258.html</a></p>
<p>Java NIO Selector详解 <a href="https://blog.csdn.net/jeffleo/article/details/54695959" target="_blank" rel="noopener">https://blog.csdn.net/jeffleo/article/details/54695959</a></p>
<p> 为什么NIO比BIO效率高 <a href="https://blog.csdn.net/wy0123/article/details/79382761" target="_blank" rel="noopener">https://blog.csdn.net/wy0123/article/details/79382761</a></p>

      
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